This invention relates to the field of fluid dynamics and, more particularly, to improved methods and compositions useful in reducing the frictional resistance encountered in the flow of aqueous liquids.
It has previously been known that various additives have the capability of reducing the extent of pressure drop resulting from energy consumed through friction between a flowing liquid and a surface, such as the inside wall of a pipe, along which the liquid is flowing. Particular applications in which such additives have been employed are water flow through long hoses used in fire fighting and in injection at the wetted surfaces of ships and torpedoes to reduce the drag resistance to passage of the vessel through the water.
An important potential application for drag reducing additives is in the proposed Alaskan pipline. If a conventional hot oil pipeline were installed underground in Alaska, it would cause melting of the permafrost and consequent subsidence and environmental damage on the north slope. Installation of such a pipeline above ground would expose the line to mechanical damage and have other adverse environmental effects, such as serving as a barrier to the migration of certain animals. To avoid these alternative problems, it has been proposed to transport crude oil from the north slope in the form of a cold oil-in-water emulsion or dispersion. The use of drag reducing additives effective in aqueous liquids carries the potential for significant reduction both in the energy requirements and maximum line pressure in an oil-in-water dispersion system.
High polymer solutes, soap solutes and suspended fibrous solids have all demonstrated the ability to reduce friction or drag in turbulent flow of aqueous liquids. High polymer additives are of limited utility, however, since they are subject to irreversible mechanical degradation in regions of high shear such as in pumps or in flow through narrow clearances. The low molecular weight degradation products are much less effective drag reducers. A further practical impediment to the use of polymer additives is presented by the very slow rates at which such additives dissolve. Weeks or even months may be required for dissolution to be completed.
Soap additives do not suffer the disadvantage of irreversible mechanical shear degradation. Mechanical degradation, is observed in such systems, but is reversible, and full drag reduction ability is regained once the solution is removed from a high stress region. Soap additives, however, do suffer from other drawbacks. Thus, metallic soaps of fatty acids are limited in their application because calcium and other cations normally present in tap water or sea water cause precipitation of insoluble soaps.
Another soap additive, which is reportedly effective as a drag reducer, is a complex soap containing equimolor amounts of cetyltrimethylammonium bromide and 1-naphthol. This soap does not precipitate in the presence of calcium ions, but its components are expensive and degrade chemically in aqueous solutions in the course of a few days.
Achievement of effective drag reduction with solid suspension requires high concentrations of solids with attendant settling and plugging problems. The drawbacks of a solid suspension system would be particularly severe in a long pipeline, especially one which traverses a remote area. Also, or course, the suspended solid must be separated from the liquid at its destination and the separation would require additional equipment with potential operational problems and yield losses.
An unfulfilled need has, therefore, existed for improved methods and compositions for reducing frictional resistance to flow in pipelines and along other surfaces.